Glass tubing has been drawn from a source of molten glass by a number of different methods, but small diameter tubing in the range of five to sixty-five millimeters OD has normally been drawn by pulling the tubing along an elongated horizontal path defined by a plurality of supporting rollers. Even though such rollers are fabricated from graphite, it is a well known fact that some deformation or marking of the surface occurs as the hot tubing is pulled horizontally over the rollers, particularly when such tubing is still plastic. This deformation increased in proportion to the production rate, since increased production results in higher forming temperatures, making the tubing more susceptible to deformation. This relationship resulted in limited production capabilities for any given alley as determined by specifications for acceptable levels of tubing deformation.
In British Pat. No. 1,025,500 issued to Corning Glass Works, there is disclosed an apparatus for supporting drawn tubing on a cushion of air. The air cushion is provided by upward air flow through a generally U-shaped porous baffle formed of sintered metal which has a diameter slightly larger than the outer diameter of the glass tubing to be supported by an amount sufficient to create an air cushion between the lower surface of the tubing and the semi-cylindrical air supplying baffle. The obvious disadvantage of this construction is the fact that it is effective for only one diameter of tubing. Those skilled in the art will recognize that any modern tubing alley has to be capable of producing a rather wide range of diameters and weights of tubing in order to supply the market needs. Since the length of the plastic range of hot glass tubing alleys can vary from twenty to fifty feet, it is obviously impractical to substitute a proper diameter porous air baffle member in the apparatus shown in the British patent each time that a change in tubing diameter is desired. For efficient operation, the downtime associated with a job change on a tubing alley must be minimized, so mechanical changes to the tube support mechanism cannot be tolerated.